1. Field of the Invention
This invention relates generally to the removal of metal ion contaminants from an aqueous solution and in particular to removal of nickel, copper, zinc and other metal ions from a trivalent chromium electroplating bath.
2. Description of Prior Art
In the electroplating industry, baths containing concentrated metal ions are utilized in the plating process. The production materials, typically referred to as substrates, are immersed in a plating bath, and electric current is applied. The plating metallic ions in the bath are plated upon the substrates. Once sufficient metal is deposited on the substrate, the substrate is removed from the bath. The substrate is then rinsed in water tanks to remove adhering, non-plated metal ions. After rinsing, the substrate is advanced to the next plating bath. Many of the substrates which are plated are metallic. For example, decorative chrome is plated over substrates of copper, nickel or brass. Tin is plated on steel, and gold is plated on copper printed circuit boards.
A typical plating bath may have concentrations of plating metal ions that range from a few thousand to hundreds of thousands of parts per million (ppm). When the substrate metals are immersed in the plating baths, small amounts of metal, typically in the form of metal ions, may be stripped from the substrate. Over time these substrate metal ion contaminants accumulate in the plating bath. Concentrations of contaminant metal ions as low as 10-100 ppm may impair the plating process. When contaminant metal ions accumulate to levels which impair the plating process, the bath is either discarded or purified. Disposal of a spent plating bath results in loss of expensive plating chemicals and presents a real waste disposal problem.
The development of an economical purification process therefore would be highly advantageous. Purification of plating baths requires separation techniques which selectively remove low concentrations of the contaminant metal ions in the presence of very high concentrations of the plating metal ions.
Use of ion-exchange resins is a possible method for plating bath purification (See for example, B.A. Bolto and L. Pawlowski, in "Wastewater Treatment by Ion Exchange", E. & R.N. Spon, New York, 1987, p. 39-48). In fact, for a number of years cation exchange resins have been used to recover metal ion contaminants from hexavalent chromium plating and anodizing baths. In this instance, hexavalent chromium (an anion as chromate or dichromate) comprises the desired species in the mother bath. Upon use, the bath becomes contaminated with such impurities as Fe.sup.+3, Mg.sup.+2, Al.sup.+3, Cu.sup.+2, Zn.sup.+2, and Ni.sup.+2 as well as Cr.sup.+3, which results from the reduction of Cr.sup.+6 to Cr.sup.+3. Although the baths are acidic with pH level usually below 1.0, a strong acid cation ion-exchange resin adsorbs the contaminating cationic metal impurities and passes the plating anions, i.e., the anionic hexavalent chromium.
Other plating baths, including pickling baths, have been purified by ion-exchange. In some metal finishing operations, acids are employed to clean the metal and strip the oxide layer from the surface of the metal. Accumulation of metal ions arising from the dissolution of the metal causes deterioration of such baths. The contaminated baths may be purified by methods similar to that outlined above for hexavalent chromium baths.
For example, magnesium sheet is pickled in an acetic acid bath. During the pickling operation, the increase in the magnesium ion Mg.sup.+2 concentration coupled with the decrease in the acetic acid concentration results in progressively slower pickling rates. The spent pickle bath, i.e., the bath contaminated with magnesium ions, is purified by passing the bath through a cation exchange resin in the protonated form. This cation exchange resin exchanges magnesium ions for protons and the acetate anion passes through the resin. This results in rejuvenation of acetic acid in the pickling bath.
The above cited examples of bath purification are based upon the separation and removal of cationic impurities from desired anionic plating species. Purification of a plating bath containing cationic plating metal ions as well as cationic metal impurities is a much more difficult problem. A specific example of such a bath is a trivalent chromium plating bath marketed by Engelhard Corporation, of Cleveland, Ohio. This trivalent chromium plating bath contains from 0.1M to 1.2M (5,100-61,300 ppm) of Cr.sup.+3 among other components, and operates at a pH between 1 and 4. (See for example, Gyllenspetz et al., U.S. Pat. No. 3,954,574 and Gyllenspetz et al., U.S. Pat. No. 4,054,494).
Once metal ion impurities such as zinc (Zn.sup.+2), copper (Cu.sup.+2) and nickel (Ni.sup.+2) accumulate in the trivalent chromium plating bath to levels between 10 and 100 ppm, the quality of chromium plating becomes impaired. Typically, a trivalent chromium bath is then either dumped or purified by use of complex precipitating agents such as ferrocyanides, which precipitate the cationic metal impurities (See for example, Crowther et al., U.S. Pat. No. 4,038,160). The bath is then filtered to remove the precipitated impurities. This process uses environmentally undesirable reagents (ferrocyanides), is time consuming, and takes the bath out of production during the purification process. Thus, an environmentally acceptable method that selectively removes contaminating metal ions from a trivalent chromium plating bath on a continuous basis without interruption of the plating process is highly desirable.
The separation of trace amounts (tens of ppm) of divalent cationic metal ion impurities from high concentrations (many thousands of ppm) of the desired trivalent chromium ion is impossible with the use of a strong acid cation exchange resin. The functional group on a strong acid cation resin is a sulphonic acid moiety which interacts with cations primarily through electrostatic attraction. Thus, this type of resin has higher affinity for the more highly charged cationic species and therefore exhibits higher affinity for trivalent cations than for divalent cations.
Hence, while environmentally and economically desirable, an on-line method for purification of a trivalent chromium plating bath contaminated with cationic impurities is not available.